In recent years, gas turbine manufacturers have become increasingly concerned with pollutant emissions. Of particular concern has been the emissions of nitrogen oxides (NO.sub.x) because such oxides are a precursor to air pollution.
It is known that NO.sub.x formation increases with increasing flame temperature and with increasing residence time. It is therefore theoretically possible to reduce NO.sub.x emissions by reducing flame temperature and/or the time at which the reacting gases remain at the peak temperatures. In practice, however, this is difficult to achieve because of the turbulent diffusion flame characteristics of present day gas turbine combustors. In such combustors, the combustion takes place in a thin layer surrounding either the evaporating liquid fuel droplets or the dispensing gaseous fuel jets at a fuel/air equivalence ratio near unity regardless of the overall reaction zone equivalence ratio. Since this is the condition which results in the highest flame temperature, relatively large amounts of NO.sub.x are produced.
It is also known that the injection of significant amounts of water or steam can reduce NO.sub.x production so that the conventional combustors can meet the low NO.sub.x emission requirements. However, such injection also has many disadvantages including an increase in system complexity, an increase in operating costs due to the necessity for water treatment, and the degrading of other performane parameters.
The problem of realizing low NO.sub.x emissions becomes even further complicated when it is necessary to meet other combustion design criteria. Among such criteria are those of good ignition qualities, good crossfiring capability, stability over the entire load range, low traverse number or flat exhaust temperature profile, long life and the ability to operate safely.
Some of the factors which result in the formation of nitrogen oxides from fuel nitrogen and air nitrogen are known and efforts have been made to adapt various combustor operations in light of these factors. See, for example, U.S. Pat. Nos. 3,958,413; 3,958,416; 3,946,553; and 4,420,929. The processes used heretofore, however, have either not been adaptable for use in a combustor for a stationary gas turbine or adequate for the reasons set fort below.
A venturi configuration can be used to stabilize the combustion flame. In such arrangements, lowered NO.sub.x emissions are achieved by lowering peak flame temperatures through the burning of a lean, uniform mixture of fuel and air. Uniformity is achieved by premixing fuel and air in the combustor upstream of the venturi and then firing the mixture downstream of the venturi sharp-edged throat. The venturi configuration, by virtue of accelerating the flow preceding the throat, is intended to keep the flame from flashing back into the premixing region. Further, the nature of the flow adjacent the downstream wall of the venturi is a zone of separated flow and is believed to serve as a flame holding region. This flame holding region is required for continuous, stable, premixed fuel burning. Because the venturi walls bound a combustion flame, they must be cooled. This is accomplished with back side impingement air which then dumps into the combustion zone at the downstream end of the venturi. However, such arrangements have not been entirely satisfactory.
U.S. Pat. No. 4,292,801 of Wilkes and Hilt, assigned to the same assignee as the present inventor, and which is hereby incorporated by reference, describes a gas turbine combustor which has an upstream combustion chamber and a downstream combustion chamber separated by a venturi throat or constriction region. Other patent applications directed at reducing the NO.sub.x emissions include application Ser. No. (51DV-2910) and application Ser. No. (51DV-2903), both of M. Kuwata, J. Waslo and R. Washam and assigned to the same assignee as the present invention, and which are hereby incorporated by reference. Application Ser. No. (51DV-2903) is directed at premixed fuel and air combustor arrangements including a venturi.
Premixed fuel combustion by its nature is very unstable. The unstable condition can lead to a situation in which the flame cannot be maintained, which is referred to as "blow-out". This is especially true as the fuel-air stoichiometry is decreased to just above the lean flammability limit, a condition that is required to achieve low levels of NO.sub.x emissions. The problem to be solved with the premixed dry low NO.sub.x combustor is to lean out the fuel-air mixture to reduce NO.sub.x while maintaining a stable flame at the desire operating temperature. Further, it is desirable to have stable premixed burning over a wide range in combustion temperature to allow for greater flexibility in operation of the gas turbine, and to increase the product life of turbine combustion systems.